The invention relates to detecting the presence of cumbustion products in the air by use of an ion chamber. Smoke particles within the ionization chamber cause a change in ion current. When the change exceeds a predetermined level, an alarm signal is triggered.
In conditons of relatively high ambient air flow velocity, i.e. above about 500 feet per minute, the performance of prior art smoke detectors of the ionization type begins to decrease with increase in flow velocity. For example, in unipolar devices, increased ambient air flow velocity typically moves the device away from alarm condition and an abnormally high level of smoke density is required to trigger an alarm; in bipolar devices, increased ambient air flow velocity moves the device toward alarm condition so a false alarm can be triggered by a small amount or even no smoke. Typically, maximum rates of ambient air flow above which the particular detector should not be employed are stated by the manufacturer.
It is an objective of this invention to provide a smoke detector of the ionization type that operates reliably at relatively high ambient air flow velocity.